RNA editing in African trypanosomes is a post-transcriptional event which results in the formation of mitochondrial mRNAs differing from their genes by the insertion or deletion of uridylyl nucleotides. In specific aim 1 the applicant proposes that RNA editing precedes by a enzyme cascade pathway initiated by the formatiom of a high molecular weight editing complex consisting of pre-edited mRNA, cognate guide RNA and associated proteins including an editing site specific endonuclease, a terminal uridylyl transferase and RNA ligase. In aim 2 they propose to examine the function of an RNA editing complex associate protein (REAP-1). In aim 3 they propose that RNA editing requires the assembly of a highly ordered complex consisting of RNAs and proteins.